The present invention relates to a cathode ray tube display apparatus having a flat configuration, for use in a television receiver, computer display terminal, etc.
The cathode ray tube (hereinafter designated as CRT) type of display is widely utilized for displaying images, characters, etc. in various applications. Various proposals have been made in the prior art for making such a CRT display apparatus more compact than has been possible hitherto, by making the overall shape of the apparatus substantially thin and flat. One such proposal is made in U.S. Pat. No. 3,723,786. The general configuration of this proposed apparatus is as shown in FIG. 1A. A photo-emissive layer 2 is formed in an inner surface of a flat transparent front portion 1 of an evacuated envelope. A vertically extending array of horizontally elongated vertical scanning deflection electrodes 3 is formed on an inner face of a rear portion 4 of the evacuated envelope, which is disposed parallel to the front portion 1 with a narrow space left between the portions 1 and 4, the vertical scanning deflection electrodes 3 being arrayed with a fixed pitch in the vertical direction. An electron gun is formed by a line cathode 5, a first grid 6, a modulation electrode 7, and a shield electrode 8, with the modulation electrode 7 being positioned between the electrodes 6 and 8 as shown. The line cathode 5, grid electrodes 6 and 8 and modulation electrode 7 are shown separately in plan views in FIG. 1B. The line cathode 5 has electron-emitting segments 5a formed at regular spacings along its length, while the electrodes 6 to 7 have respective rows of apertures 6a, 7a, 8a formed therein with a common pitch which is identical to that of the electron-emitting portions 5a of the line cathode 5. The electrodes 6 and 8 are electrically conductive, and are connected to respective fixed potentials, while the modulation electrode 7 is formed of an electrically non-conductive material and has an "eyelet" of conducting material formed around the periphery of each aperture 7a, with respective modulation signlas being applied to these eyelets. The picture element 2 is connected to a high voltage V.sub.D, so that a horizontally extending set of vertically directed electron beams are emitted through the apertures in the electrodes 6 to 8 and into the space between the picture element 2 and the vertical scanning deflection electrodes 3, as indicated by the upwardly extending broken line in FIG. 1A. By applying the voltage V.sub.D to a lower set of the vertical scanning deflection electrodes 3 and a more negative voltage (V.sub.D -V.sub.C) to an upper set of the vertical scanning deflection electrodes as shown, all of the electron beams are deflected towards the picture element 2, when they reach the first vertical scanning deflection electrode which is connected to the (V.sub.D -V.sub.C) potential. Thus, raster scanning of such a display device can be performed applying sequential scanning voltages to the vertical scanning deflection electrodes 3 while modulating the respective intensities of the electron beams, so that an image such as a television picture can be displayed, if the number of the vertical scanning deflection electrodes is made substantially equal to the number of horizontal scanning lines of the display picture and the number of apertures of each of the electrodes 6 to 8 is made substantially equal to the number of picture elements of the display picture.
However such a display apparatus has some serious disadvantages. Firstly, since the number of electron beams that must be produced by the electron gun is equal to the number of picture elements in each horizontal scanning line of the display image, a very large number of electron beams must be produced which are positioned with very close mutual spacings. In the case of a usual CRT display television receiver, the pitch of the picture elements along the horizontal direction is approximately 0.1 to 0.2 mm. In order to generate a set of electron beams which are separated by such a small pitch, and to modulate these beams, it is necessary to execute very precise machining operations to form the components of the electron gun, while in addition practical problems will arise with forming electrical connections to the modulation electrode 7, for applying modulation signals for the respective electron beams.
Furthermore, since a substantial distance is traversed by each electron beam from the point of leaving the electron gun (in which beam focusing and directing is implemented) to the point of incidence upon the picture element 2, it is difficult to avoid errors in the respective positions at which the beams fall upon the picture element 2. In addition, it is difficult to ensure that emitted-light spots of uniform diameter are produced by the electron beams over the entire display area. Variations in spot diameter will result in corresponding variations in display luminance, while in the case of a color television display, variations in spot size or errors in beam landing position will result in color errors in the displayed image. Thus, considerable problems would result if it were attempted to use such a prior art display apparatus to display characters or images with a normally acceptable level of display resolution.
Another problem arises with such a prior art display apparatus, which is not significant when the display area is small. That is, in the case of a large display area, the length of the cathode of the electron gun will be of substantial length. In general, such a cathode is formed as a line cathode, which is supported under tension by springs at each end such as to be supported in a free-floating condition. Thus, such a cathode is very susceptible to the effects of vibration, which can result in instability of the level of electrons emitted from the cathode. This produces visible noise on the display, and also varies the electron beam landing positions, so that a reliably clear display cannot be ensured.